1. Field of the Invention
This invention relates to the production of urea from carbon dioxide and ammonia, and more particularly it relates to a process for efficiently separating and recovering unreacted excess ammonia and ammonium carbamate from a urea synthesis effluent.
2. Description of the Prior Art
As is well known in the art, the urea synthesis effluent which is obtained by reacting carbon dioxide with a stoichiometric excess of ammonia under elevated temperature and pressure conditions contains unreacted ammonium carbamate and an excess of ammonia (hereinafter referred to briefly as "unreacted materials") apart from urea and water. In order to effect urea synthesis in an efficient manner, it is necessary to separate these unreacted materials from the urea synthesis effluent and to recycle the thus separated materials to the urea synthesis zone. In general, the unreacted materials can be readily separated by heating the urea synthesis effluent to gasify the unreacted materials. The higher the temperature and the lower the pressure, the separation of the unreacted materials from the urea synthesis effluent is feasible with more ease from the viewpoint of equilibrium. However, when the effluent is heated to too high a temperature, urea tends to be hydrolyzed and the amounts of further impurities such as biuret, etc., are increased. Accordingly, upon separation, the pressure of the urea synthesis effluent is usually reduced and the separation is effected at a low temperature corresponding to the reduced pressure.
In order to recycle the separated unreacted materials comprising the excess ammonia, carbon dioxide and water to the urea synthesis zone, they may be merely compressed and recirculated in gaseous form or they may be condensed to a liquid and recirculated. However, the former method involves several problems such as the requirement of a great deal of power upon compression, mechanical difficulties in compression of the hot and corrosive gas, etc. On the other hand, the latter method is free of the above problems but has its own disadvantage in that the heat released upon condensation of the separated gaseous mixture of unreacted materials cannot be recovered and used effectively since the condensation temperature is low. In addition, when all of the separated unreacted materials are condensed, the condensed liquid is readily separated into two liquid layers, i.e., an ammonia-rich layer and an ammonium carbamate-rich layer, involving some difficulty upon recycling to the urea synthesis zone by means of a pump. In order to avoid this difficulty, it is required that the greater part of the ammonia is separated from the unreacted materials and condensed in an additional condenser for the ammonia.